1. Why Ceramic Dishes Are Hard and Heat-Resistant but Cannot Bend
Ceramics are made of atoms linked by strong bonds. Because of these strong bonds, atoms cannot slide to a neighboring position even when a large force is applied. The material may flex elastically, but it does not deform permanently and instead breaks. This is a major difference from metals, where atoms can shift positions to allow bending without breaking [1].
Another factor in how dishes break is the presence of tiny flaws. Microscopic scratches on the surface or inside the material act as starting points for cracks. These flaws do not mean the product is defective; they are just tiny scratches, and cracks spread from them. Because there is no permanent deformation, the strength of a dish is determined by its weakest point, which is often where a flaw is located. As a result, even dishes of the same shape can vary in how easily they break [2].
2. Why Hot Soup Does Not Harm Ceramic Dishes
Ceramic dishes are fired at high temperatures, around 1200°C (about 2192°F), during production. The firing temperature is a separate scale from how hot food is at the table, and it is not the only basis for heat resistance. Usable temperatures and cooking methods vary by product [3].
Because the atomic bonds in ceramics are strong, these materials tend to have high melting points. For example, aluminum melts at about 660°C (about 1220°F). In contrast, alumina, a type of fine ceramic, does not melt or decompose until temperatures exceed 2000°C (about 3632°F) [4]. It is important to note that these figures apply to fine ceramics, not necessarily to everyday ceramic dishes. The heat resistance of a dish is not determined solely by how high a temperature it can withstand, as the next section explains.
3. Why Sudden Temperature Changes Can Break Dishes
A main cause of breakage in ceramic dishes is a sudden change in temperature. When one side of a dish heats up quickly, it expands. The cooler side does not expand at the same rate, creating a strong pulling force that can break the material. This force is proportional to the temperature difference and the material's thermal expansion coefficient, which measures how much it expands per degree of temperature rise. Materials with a low thermal expansion coefficient experience less pulling force during temperature changes. Actual resistance to thermal shock also depends on the material's strength [5].
For comparison, the thermal expansion coefficient of iron is 12, copper is 17, and aluminum is 23 (in ppm per °C; ppm means parts per million). A ceramic called cordierite has a coefficient of about 1 ppm/°C. Because cordierite barely changes length with temperature, the forces created by the difference in expansion and contraction are small. The same source gives the example of a glass cup cracking when hot water is poured into it [5]. Note that ceramics in general do not have low expansion; this example is a property of cordierite. For fine ceramics, thermal shock resistance is expressed as the temperature difference at which the material breaks when cooled rapidly. Silicon nitride is given as an example that withstands a 550°C difference in water cooling [7]. Going from cold to hot suddenly, or heating only part of the dish, increases the risk. Not all ceramic dishes are safe for direct fire [3].
4. How Petalite Makes Ceramic Pots Heat-Resistant
Some heat-resistant earthenware uses petalite, a lithium-containing mineral. It is used to form crystals with low thermal expansion in the clay body, which reduces the expansion of the pot [6].
Petalite-based heat-resistant ceramics were reportedly developed and mass-produced around 1959. Since then, improvements to the clay body and the development of glazes have continued [6]. A report from the Mie Prefectural Industrial Research Institute lists thermal expansion coefficients measured at 600°C (about 1112°F). Semi-porcelain clay had a coefficient of 7.93, and tile clay was 8.92. Clay from commercial earthenware pots measured 2.58 and 2.16, while experimental clay designed for low expansion measured 0.87. The clays containing petalite had small values. Note that these were measured at 600°C, not at room temperature [6].
In a thermal shock test, the petalite-containing earthenware-pot clay kept its bending strength at temperature differences up to 600°C, and did not change after five repeats of a 600°C difference. The ordinary ceramic clays tested (semi-porcelain and tile) lost strength as the temperature difference increased. These results come from tests on rod-shaped samples. They do not mean that a real earthenware pot can safely handle a 600°C temperature difference in actual use.
5. Why a "Heat-Resistant" Label Does Not Mean Every Use Is Safe
Just because a dish is labeled "heat-resistant" does not mean it can be used for everything. The allowed uses are specific to each product. Labels on the dish or its box indicate whether it is safe for direct fire, ovens, or microwaves.
In tests for heat-resistant tableware, dishes are cooled rapidly from a set temperature difference. Then, the dish is checked by eye for cracks or peeling in the glaze, and for cracks or breaks in the clay body [6].
This test follows the JIS S 2400 standard for heat-resistant ceramic tableware.
6. How to Check the Labels on Your Own Dishes
If you want to investigate further, you can look at the dishes in your home. Choose a few and check the bottom or the box for labels indicating "microwave," "oven," or "direct fire." Compare dishes with labels to those without, looking for differences in material or thickness. Do not try to break them or heat them suddenly to test them.
For official information, explanations from NGK Ceramic Academy and Kyocera are available in Japanese. Always follow the instructions and labels on the specific product you purchased.